Where is that located? Can you take a picture of it pleaseGo to the dealership. This is how my car got fixed when it had the same noise. a rubber gasket wasn't in its place, and because of that, it wasn't sealing properly against the noise. good luck!
I had a similar issue with a small gap in the window, and it caused a whistling sound, particularly at higher speeds. The noise would get louder as I accelerated, and it was really annoying. Had a technician fix itIsn;t that just a wind noise? How fast are you driving? The most common cause is air leaking through gaps in the mirror housing or around the seal where the mirror attaches to the car. Even a tiny gap can create a whistle at higher speeds.
I used the same approach with mine to quickly identify the problem. Hopefully the OP has managed to resolve itI've heard similar things with other cars. It's usually wind hitting a small gap or edge. Maybe try taping off different parts of the mirror with masking tape to see if you can pinpoint where it's coming from?